The present invention relates generally to repeaters. More particularly, it concerns a digital repeater having a digital signal processor that is software configurable for receiving and retransmitting radio frequency signals.
Repeater systems are typically used where the area to be covered and/or capacity requirements do not justify the installation of a full base station system with managed frequency-channel allocation. Thus, tower-mounted repeater systems are often used to extend the range of a base station and to fill nulls in the coverage area of the base station. Nulls are regions that are blocked from receiving radio frequency (RF) signals. Nulls include hills, trees, buildings, etc.
The typical repeater system comprises three basic parts: a link antenna which is directed/aimed at the base station antenna; repeater electronics; and a broadcast antenna which is directed towards the area of interest. Often, the link antenna is highly directive (high gain) with a very narrow beamwidth because it only needs to xe2x80x9cseexe2x80x9d the base station antenna. The broadcast antenna has a larger beamwidth which is determined by the intended area to be covered. The repeater electronics may contain an assortment of diplexers, filters, splitters, and RF amplifiers. A typical repeater system includes a forward path from the base station (BS) antenna to a receiving apparatus and a reverse path from the receiving apparatus to the BS antenna.
Two primary performance factors on which an antenna system is based are gain and output power. The output power is mostly determined by the sum of the link and broadcast antenna gains and the maximum (linear) output power of the amplifier(s). The system gain is determined by the sum of the passive antenna gains, plus the gain of the amplifier(s). This is limited by the isolation (or mutual coupling) between the broadcast and link antennas. The isolation depends on the antenna type, front to back (F/B) ratio and beamwidth.
Repeater systems are used in a variety of applications such as TV and radio transmission, cellular/PCS communications and pager services. Existing repeaters use analog technology and typically only provide one channel for the forward path and one channel for the reverse path. To obtain significant adjacent channel rejection, the RF signal must be converted to a fixed Intermediate Frequency (IF). Thus, each channel in a traditional repeater requires a dedicated down converter, a dedicated IF filter, a dedicated up converter and a dedicated synthesizer. A traditional analog repeater uses a Local Oscillator (LO) having a resolution of 12.5 kHz to 200 kHz. This sets the channel resolution of the entire repeater. The IF and bandwidth of an analog repeater are fixed based on the modulation format for which the repeater is designed. In addition, analog repeaters suffer from group delay and passband variations due to part-to-part variations in the IF surface acoustic wave (SAW) filter or crystal filter. Small impedance mismatches can create significant ripple in the passband response. In an analog repeater the shape factor of a SAW filter may have a 200 kHz to 400 kHz frequency difference between the 15 dB and 40 dB points. The passband response of an analog repeater is fixed by the IF SAW filter or crystal filter.
Therefore, there is a need for a repeater capable of: transmitting and receiving multiple channels which uses only a single RF down converter/RF up converter pair for each path, greater local oscillator resolution, processing multiple modulation formats without changing the repeater hardware, virtually identical filter performance despite impedance mismatches, allowing more channels to be placed closer together due to filters that have a shape factor with less than 1 kHz frequency difference between the 15 dB and 40 dB points, matching the modulation format used by the communications network, and automatically detecting the modulation format and changing the filter bandwidth based on the modulation format detected. The present invention is directed to addressing one or more of these needs.
A digital repeater for transmitting and receiving radio frequency (RF) signals. The digital repeater includes a down converter for down converting a first RF signal to an intermediate frequency (IF) signal. An analog-to-digital converter for converting the to IF signal into a digital signal. A digital signal processor for filtering and amplifying the digital signal. A digital-to-analog converter for converting the digital signal into an analog signal. The digital repeater further includes an up converter for up converting the analog signal to a second RF signal suitable for antenna transmission.